This chapter focuses on the mucus layer of the intestinal tract and its role in colonization of the intestine by enteric bacteria. The gel-like mucus layer of the intestine is dynamic in that it is continuously being renewed by secretion of stored or newly synthesized components, sloughed or eroded by mechanical forces, and degraded by the indigenous flora. In addition to the major gel-forming mucins present, the mucus layer contains a variety of proteins, carbohydrates, lipids, nucleic acids, and other components that originate from a number of sources. The presence of the intestinal mucus layer as a viscous physical barrier and the need for bacteria to reach and maintain themselves in an appropriate environmental niche has led many investigators to propose chemotaxis and motility as important aspects of colonization for the microorganisms that exhibit these properties. In the case of the normal flora of the large intestine, association with the mucosal surface appears to be a critical aspect of colonization and, depending on the organism, may or may not involve direct adhesion to epithelial cells. The lumen of the upper intestine may contain a number of microorganisms, but the rate of bacterial growth in the lumen in these regions does not does not appear to be high enough to compensate for the rapid flow of luminal contents. The types of experiments being used to elucidate the carbon nutrition for colonization of Escherichia coli can and should be extended to include other members of the intestinal microflora.

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